When performing positioning in an indoor environment, determining a region, including disambiguating between different regions of the indoor environment is useful, if not critical. Indoor regions may be different floors of a building or portions of floors in a building or different portions of a floor. In this case, signals from wireless transceiver access points (APs) in different regions can be received by a single mobile station (MS). Even though the MS is in a first region, the signal strength received from an AP in a second, different, region may be stronger than the signal strength of a signal received from an AP in the first region. This is especially true near portals such as staircases and elevator shafts, e.g., due to waveguide effects of these structures. Consequently, region determination is performed to determine in which region the MS resides. Region determining, including disambiguating between different indoor regions of a structure, can be a time-consuming, highly power-consuming process.